The donation was announced Tuesday by Washington United for Marriage, which is working to approve Referendum 74. The donation was made on Oct. 15, but didn’t publicly post online with the Public Disclosure Commission until Tuesday. Referendum 74 asks voters to either approve or reject the law allowing same-sex marriage that was passed by the Legislature earlier this year.

Bill Gates had made a $100,000 contribution to the campaign earlier this year.

About $11 million has been spent on the campaign so far, with a bulk of it spent by gay marriage supporters. Washington United for Marriage has far outraised its opponents, bringing in nearly $11 million compared to the more than $2 million raised so far by Preserve Marriage Washington, which opposes the law.